This invention relates to electronic assemblies, and, more particularly, to the dissipation of accumulated charges and the protection of such electronic assemblies.
In one common architecture, electronic devices are assembled onto substrates, forming electronic subsystems. The subsystems are assembled into a chassis or other housing which, with appropriate interconnections and external connections, forms the electronic system. A well-known version of this architecture is the printed wiring board (PWB) or printed circuit board (PCB) widely used in consumer electronics.
PWBs are sometimes used in applications which may be subjected to adverse environments, such as in military or space applications. It is known to protect the PWB, including the circuitry and electronic components thereon, with a conformal coating. This conformal coating seals and protects the PWB circuit against adverse external influences such as foreign objects, moisture, corrosion, and solvents. The conformal coating aids in the mechanical support of the electronic components and helps protect the PWB circuitry against thermal shock.
The conformal coating also electrically insulates the PWB. In many applications, such electrical insulation of the PWB is desirable in that it reduces the risk of electrical shorting by foreign objects which may contact the PWB. In others, it may be undesirable because it results in charge accumulation which, in turn, can cause arcing or other electrical damage to the components on the PWB. In space applications, for example, the PWB is subjected to a flux of electrons during service. The electrons remain generally stationary on the PWB, resulting in charge accumulation. The PWB is therefore typically electrically grounded in such applications, although the grounding may not be fully effective because there still may be harmful charge accumulations in some regions of the PWB.
Two types of conformal coatings are in widespread use today. One is the standard MIL-I-46058 insulating coating, which provides for no dissipation of the accumulated charge. The other is a two-layer coating in which one layer is sufficiently electrically conductive to dissipate accumulated static charge. The two-layer coating, while operable to some degree, suffers from the shortcomings that it must be applied very carefully to avoid inhomogeneities that interrupt the charge dissipation, and it may also act as a capacitor to interfere with the operation of the electronic components.
There is a need for an improved conformal coating for electronic devices and substrates. The present invention fulfills this need, and further provides related advantages.